Loco Differences

[Jordan6]

Just curious as to what the differences, if any, there are between a Dash 9 and a AC4400. They both look the same to me, and they both have the same 4390 HP engines, so what am I missing here.[:)]

[BaltACD]

The Dash 9's use DC traction technology.

The AC44's use AC traction technology.

[Jordan6]

Thanks for your reply, but, now I'm wondering how the AC differs from the DC locomotives.[%-)] Why would a railroad choose one over the other? Could it be maintance, durability, traction, geography of the railroad, or even the average climate conditions?

[csxengineer98]

the AC locos are also have a -9 desigintion.... such as the CW-44-9AC..and the CW-44-9
the AC locos have no short time ratins on the traction motors...meaning you can be in the 8th notch and loading over 1000 amps..and not damage the traction motors....a DC traction motor had a time limit in which you can run the traction motors at high amps (in the red zone) after the time limet expires in a short time rating zone on the load meter...the locomotive must be stoped...the genorator field switch opend...the reverser centered...and the throttle set at i think it was notch 3 or 4 for about 20 mins to alow the blowers to cool the traction motors down..befor your alowed to proseed agin... and after that 20 min cool down time...should you go into short time ratings agin... and after that new time limet passes..you have to stop and cool down agin....
csx engineer

[M636C]

Jordan,

The way to tell a DC Dash 9 from an AC 4400 is to check out the left side of the locomotive behind the cab. On a Dash 9 there is a large box with the cab air conditioner mounted on top. On the AC4400 there is a really large box which contains the AC inverters (six, one for each axle). There is no room for the air conditioner, and this is placed under the cab (where the electrical switch gear is located in the Dash 9).
The AC4400 also has an additional dynamic brake fan in the hood behind the cab located near the central air intake. This should show up clearly on models if these are easier to find to compare!

Peter

[daveklepper]

Generally, AC locomotives have it all over dc in heavy hauling where work where high tractive effort is needed for a long time, keeping freight at speed up mountains and braking continuously going downhill. Also, being free of commutators and bruxhes, the ac motors need lots less maintenance. Otherwise, say for high speed level ground operations, where tractive effort is required for starting the train but much less to maintain speed, operationally the dc locomotive can perform as well as the ac. But still, the ac requires less maintenance. First cost about 20% more though. Dave

[tree68]

I recall reading a while back about a drag somewhere that was running in notch 8 at a fractional MPH (<1) with AC locos. They'd have kept going, but they parted a coupler. The article said such a performance would not have been possible with DC.

[wabash1]

csxengineer

the short time rating you talk about is true the way they have us do the cool down is notch 8 for 10 min . again this is for ge engines as they will shut down the power to the axels where a emd will let you go. if i am 200ft from topping a grade im going for it. there is always plenty of time to get the motors cooled down on the back side of the hill. and still be moving along. with out damage.

[Jordan6]

I've never understood what the differences are between AC/DC locomotives, but you all have given me a better understanding of how they operate. Thanks everyone[swg]

[tree68]

QUOTE: Originally posted by Jordan6

I've never understood what the differences are between AC/DC locomotives, but you all have given me a better understanding of how they operate. Thanks everyone[swg]

To get a touch technical:
DC locos are just like a model - To go faster, you apply more volts and amps. With DC, the voltage is essentially constant. If you were to look at it on an oscillograph, the voltage would be a straight line. Below a certain speed, efficiency starts to suffer. At some point, the motors stall, but they are still getting 1000 amps at 600 volts, hence the potential overheat problem.

Many of today's "DC" locos actually have the diesel turning an alternator, generating alternating current (AC), which is then rectified to Direct Current (DC) to turn the traction motors.

AC locos use more sophisticated circuitry to shape the voltage into "waves" (actually "square waves" - they look like the top of a castle) to send to the AC traction motors. It is similar in principal to the "pulsed" voltage used in some model railroad throttles. Because of the control offered by this shaping, there is finer control of the motors, allowing very low speeds with very high torque.

A bit simplistic, but maybe it gives you a little more to work with...[8D]